Porous membranes and methods for using same

ABSTRACT

A porous membrane comprising a porous substrate and a tin oxide-containing material in contact with at least a portion of the porous substrate.

RELATED APPLICATIONS

This application is a division of Ser. No. 348,789 filed May 8, 1989,now U.S. Pat. No. 5,167,820, which is a continuation-in-part ofco-pending applications Ser. Nos. 272,517 and 272,539, each filed Nov.17, 1988 and now abandoned, each of which applications is acontinuation-in-part of application Ser. No.082,277, filed Aug. 6, 1987,now U.S. Pat. No. 4,787,125 which application, in turn, is a division ofapplication Ser. No. 843,047, filed Mar. 24, 1986, now U.S. Pat. No.4,713,306. These earlier filed applications and these U.S. Patents areincorporated in their entireties herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a process for coating a substrate. Moreparticularly, the invention relates to coating a substrate with a tinoxide-containing material, preferably an electronically conductive tinoxide-containing material.

Even though there has been considerable study of alternativeelectrochemical systems, the lead-acid battery is still the battery ofchoice for general purposes, such as starting an automotive vehicle,boat or airplane engine, emergency lighting, electric vehicle motivepower, energy buffer storage for solar-electric energy, and fieldhardware, both industrial and military. These batteries may beperiodically charged from a generator.

The conventional lead-acid battery is a multi-cell structure. Each cellcomprises a set of vertical positive and negative plates formed oflead-acid alloy grids containing layers of electrochemically activepastes. The paste on the positive plate when charged comprises leaddioxide, which is the positive active material, and the negative platecontains a negative active material such as sponge lead. An acidelectrolyte, based on sulfuric acid, is interposed between the positiveand negative plates.

Lead-acid batteries are inherently heavy due to use of the heavy metallead in constructing the plates. Modern attempts to produce light-weightlead-acid batteries, especially in the aircraft, electric car andautomotive vehicle fields, have placed their emphasis on producingthinner plates from lighter weight materials used in place of and incombination with lead. The thinner plates allow the use of more platesfor a given volume, thus increasing the power density.

Higher voltages are provided in a bipolar battery including bipolarplates capable of through-plate conduction to serially connectedelectrodes or cells. The bipolar plates must be impervious toelectrolyte and be electrically conductive to provide a serialconnection between electrodes.

U.S. Pat. Nos. 4,275,130; 4,353,969; 4,405,697; 4,539,268; 4,507,372;4,542,082; 4,510,219; and 4,547,443 relate to various aspects oflead-acid batteries. Certain of these patents discuss various aspects ofbipolar plates.

Attempts have been made to improve the conductivity and strength ofbipolar plates. Such attempts include the use of conductive carbonparticles or filaments such as carbon, graphite or metal in a resinbinder. However, such carbon-containing materials are oxidized in theaggressive electrochemical environment of the positive plates in thelead-acid cell to acetic acid, which in turn reacts with the lead ion toform lead acetate, which is soluble in sulfuric acid. Thus, the activematerial is gradually depleted from the paste and ties up the lead as asalt which does not contribute to the production or storage ofelectricity

The metals fare no better; most metals are not capable of withstandingthe high potential and strong acid environment present at the positiveplates of a lead-acid battery. While some metals, such as platinum, areelectrochemically stable, their prohibitive cost prevents their use inhigh volume commercial applications of the lead-acid battery.

One approach that shows promise of providing benefits in lead acidbatteries is a battery element, useful as at least a portion of thepositive plates of the battery, which comprises an acid resistantsubstrate coated with a stable doped tin oxide.

The combination of an acid resistant substrate coated with doped tinoxide has substantial electrical, chemical, physical and mechanicalproperties making it useful as a lead-acid battery element. For example,the element has substantial stability in the presence of, and isimpervious to, the sulfuric acid or the sulfuric acid-based electrolyte.The doped tin oxide coating on the acid resistant substrate provides forincreased electrochemical stability and reduced corrosion in theaggressive, oxidative-acidic conditions present on the positive side oflead-acid batteries.

Another application where substrates with coatings, e.g., electronicallyconductive coatings, find particular usefulness is in the promotion ofchemical reactions, e.g., gas/liquid phase reactions, electro catalyticreactions, photo catalytic reactions, redox reactions, etc. As anexample of a type of reaction system, a catalytic, e.g., metallic,component is contacted with the material to be reacted, e.g., nitrogenoxides to be reduced, and a reducing gas is passed through or near tothe catalytic component to enhance the chemical reaction, e.g., thenitrogen oxide reduction to nitrogen. In addition, using a substrate forthe catalytic component which is coated with an electronicallyconductive material is highly advantageous for electro catalysis sincean electronic field/current can be effectively and efficiently providedto or near the catalytic component for electron transfer reactions. Manytypes of chemical reactions can be advantageously promoted using coatedsubstrates. Tin-oxide containing coatings on substrates may promoteelectron transfer whether or not the chemical reaction is conducted inthe presence of an electrical current or field. In addition, tin oxidecoated substrates and sintered tin dioxides are useful as gas sensors,as gas purifiers, as flocculants and as filter medium components. One ormore other components, e.g., metal components, are often included incertain of these applications.

In many of the above-noted applications it would be advantageous to havean electronically conductive tin oxide which is substantially uniform,has high electronic conductivity, and has good chemical properties,e.g., morphology, stability, etc.

A number of techniques may be employed to provide conductive tin oxidecoatings on acid resistant substrates. For example, the chemical vapordeposition (CVD) process may be employed. This process comprisescontacting a substrate with a vaporous composition comprising a tincomponent and a dopant-containing material and contacting the contactedsubstrate with an oxygen-containing vaporous medium at conditionseffective to form the doped tin oxide coating on the substrate.Conventionally, the CVD process occurs simultaneously at hightemperatures at very short contact times so that tin oxide is initiallydeposited on the substrate. However tin oxide can form off the substrateresulting in a low reagent capture rate. The CVD process is well knownin the art for coating a single flat surface which is maintained in afixed position during the above-noted contacting steps. The conventionalCVD process is an example of a "line-of-sight" process or a "twodimensional" process in which the tin oxide is formed only on thatportion of the substrate directly in the path of the tin source as tinoxide is formed on the substrate. Portions of the substrate which areshielded from the tin oxide being formed, e.g., such as pores whichextend inwardly from the external surface and substrate layers which areat least partially shielded from the depositing tin oxide by one or moreother layers closer to the external substrate surface, do not getuniformly coated, if at all, in a "line-of-sight" process. A particularproblem with "line-of-sight" processes is the need to maintain a fixeddistance between the tin source and the substrate. Otherwise, tindioxide can be deposited or formed off the substrate and lost, with acorresponding loss in process and reagent efficiency.

One of the preferred substrates for use with batteries, such aslead-acid batteries, are glass fibers, in particular a porous mat ofglass fibers. Although the CVD process is useful for coating a singleflat surface, for the reasons noted above this process tends to producenon-uniform and/or discontinuous coatings on woven glass fiber mats.Such non uniformities and/or discontinuities are detrimental to theelectrical and chemical properties of the coated substrate. A newprocess, e.g., a "non-line-of-sight" or "three dimensional" process,useful for coating such substrates would be advantageous. As usedherein, a "non-line-of-sight" or "three dimensional" process is aprocess which coats surfaces of a substrate with tin oxide whichsurfaces would not be directly exposed to vaporous tin oxide-formingcompounds being deposited on the external surface of the substrateduring the first contacting step. In other words, a "three dimensional"process coats coatable substrate surfaces which are at least partiallyshielded by other portions of the substrate which are closer to theexternal surface of the substrate, e.g., the surfaces of the internalfibers of a porous mat of glass fibers.

In "Preparation of Thick Crystalline Films of Tin Oxide and Porous GlassPartially Filled with Tin Oxide" , by R. G. Bartholomew et al, J.Electrochem, Soc. Vol 116, No. 9, p 1205(1969), a method is describedfor producing films of SnO₂ on a 96% silica glass substrate by oxidationof stannous chloride. The plates of glass are pretreated to removemoisture, and the entire coating process appears to have been done underanhydrous conditions. Specific electrical resistivity values for SnO₂-porous glass were surprisingly high. In addition, doping with SbCl₃ wasattempted, but substantially no improvement, i.e., reduction, inelectrical resistivity was observed. Apparently, no effective amount ofantimony was incorporated. No other dopant materials were disclosed.

In "Physical Properties of Tin Oxide Films Deposited by Oxidation ofSnCl₂ " , by N. Srinivasa Murty et al, Thin Solid Films, 92(1982)347-354, a method for depositing SnO₂ films was disclosed which involvedcontacting a substrate with a combined vapor of SnCl₂ and oxygen.Although no dopants were used, dopant elements such as antimony andfluorine were postulated as being useful to reduce the electricalresistivity of the SnO₂ films.

This last described method is somewhat similar to the conventional spraypyrolysis technique for coating substrates. In the spray pyrolysisapproach, tin chloride dissolved in water at low pH is sprayed onto ahot, i.e., on the order of about 600° C., surface in the presence of anoxidizing vapor, e.g., air. The tin chloride is immediately converted,e.g., by hydrolysis and/or oxidation, to SnO₂, which forms a film on thesurface. In order to get a sufficient SnO₂ coating on a glass fibersubstrate to allow the coated substrate to be useful as a component of alead-acid battery, on the order of about 20 spraying passes on each sidehave been required. In other words, it is frequently difficult, if notimpossible, with spray pyrolysis to achieve the requisite thickness anduniformity of the tin oxide coating on substrates, in particular threedimensional substrates.

Dislich, et al U.S. Pat. No. 4,229,491 discloses a process for producingcadmium stannate layers on a glass substrate. The process involvesdipping the substrate into an alcoholic solution of a reaction productcontaining cadmium and tin; withdrawing the substrate from the solutionin a humid atmosphere; and gradually heating the coated substrate to650° C. whereby hydrolysis and pyrolysis remove residues from the coatedsubstrate. Dislich, et al is not concerned with coating substrates forlead-acid batteries, let alone the stability required, and is notconcerned with maintaining a suitable concentration of a volatiledopant, such as fluoride, in the coating composition during productionof the coated substrate.

Pytlewski U.S. Pat. No. 4,229,491 discloses changing the surfacecharacteristics of a substrate surface, e.g., glass pane, by coating thesurface with a tin-containing polymer. These polymers, which may containa second metal such as iron, cobalt, nickel, bismuth, lead, titanium,vanadium, chromium, copper, molybdenum, antimony and tungsten, areprepared in the form of a colloidal dispersion of the polymer in water.Pytlewski discloses that such polymers, when coated on glass surfaces,retard soiling. Pytlewski is not concerned with the electricalproperties of the polymers or of the coated substrate surfaces.

Gonzalez-Oliver, C.J.R. and Kato, I. in "Sn (Sb)-Oxide Sol-Gel Coatingsof Glass" , Journal of Non-Crystalline Solids 82(1986) 400-410North-Holland, Amsterdam, describe a process for applying anelectrically conductive coating to glass substrates with solutionscontaining tin and antimony. This coating is applied by repeatedlydipping the substrate into the solution or repeatedly spraying thesolution onto the substrate. After each dipping or spraying, the coatedsubstrate is subjected to elevated temperatures on the order of 550°C.-600° C. to fully condense the most recently applied layer. Otherworkers, e.g., R. Pryane and I. Kato, have disclosed coating glasssubstrates, such as electrodes, with doped tin oxide materials. Theglass substrate is dipped into solution containing organo-metalliccompounds of tin and antimony. Although multiple dippings are disclosed,after each dipping the coated substrate is treated at temperaturesbetween 500° C. and 630° C. to finish off the polycondensationreactions, particularly to remove deleterious carbon, as well as toincrease the hardness and density of the coating.

Although a substantial amount of work has been done, there continues tobe a need for a new method for coating substrates with doped tin oxide.

SUMMARY OF THE INVENTION

A new process for at least partially coating a substrate with a dopedtin oxide-forming material has been discovered. In brief, the processcomprises contacting the substrate with stannous chloride, in a vaporousform and/or in a liquid form, to form a stannous chloride-containingcoating on the substrate; contacting the substrate with a fluorinecomponent, i.e., a component . containing free fluorine and/or combinedfluorine (as in a compound), to form a fluorine component-containingcoating on the substrate; and contacting the thus coated substrate withan oxidizing agent to form a fluorine doped tin oxide, preferably tindioxide, coating on the substrate. The contacting of the substrate withthe stannous chloride and with the fluorine component can occurtogether, i.e., simultaneously, and/or in separate steps.

This process can provide coated substrates which have substantialelectronic conductivity so as to be suitable for use as components inbatteries, such as lead-acid storage batteries. Substantial coatinguniformity, e.g., in the thickness of the tin oxide-containing coatingand in the distribution of dopant component in the coating, is obtained.Further, the present fluorine doped tin oxide coated substrates haveoutstanding stability, e.g., in terms of electrical properties andmorphology, and are thus useful in various applications. In addition,the process is efficient in utilizing the materials which are employedto form the coated substrate.

DETAILED DESCRIPTION OF THE INVENTION

In one broad aspect, the present coating process comprises contacting asubstrate with a composition comprising tin chloride forming components,including stannic chloride, stannous chloride and mixtures thereof,preferably stannous chloride, at conditions, preferably substantiallynon-deleterious oxidizing conditions, more preferably in a substantiallyinert environment or atmosphere, effective to form a stannouschloride-containing coating on at least a portion of the substrate. Thesubstrate is also contacted with at least one fluorine component atconditions, preferably substantially non-deleterious oxidizingconditions, more preferably in a substantially inert atmosphere,effective to form a fluorine component-containing coating on at least aportion of the substrate. This substrate, including one or more coatingscontaining tin chloride, preferably stannous chloride, and fluorinecomponent, is contacted with at least one oxidizing agent at conditionseffective to convert tin chloride to tin oxide and form a fluorine dopedtin oxide, preferably tin dioxide, coating on at least a portion of thesubstrate. By "non-deleterious oxidation" is meant that the majority ofthe oxidation of stannous chloride coated onto the substrate takes placein the oxidizing agent contacting step of the process, rather than inprocess step or steps conducted at non-deleterious oxidizing conditions.The process as set forth below will be described with reference tostannous chloride, which has been found to provide particularlyoutstanding process and product properties.

The fluorine component-containing coating may be applied to thesubstrate before and/or after and/or during the time the substrate iscoated with stannous chloride. In a particularly useful embodiment, thestannous chloride and the fluorine component are both present in the samcomposition used to contact the substrate so that the stannouschloride-containing coating further contains the fluorine component.This embodiment provides processing efficiencies since the number ofprocess steps is reduced (relative to separately coating the substratewith stannous chloride and fluorine component). In addition, therelative amount of stannous chloride and fluorine component used to coatthe substrate can be effectively controlled in this "single coatingcomposition" embodiment of the present invention.

In another useful embodiment, the substrate with the stannouschloride-containing coating and the dopant component-containing coatingis maintained at conditions, preferably at substantially non-deleteriousoxidizing conditions, for a period of time effective to do at least oneof the following: (1) coat a larger portion of the substrate withstannous chloride-containing coating; (2) distribute the stannouschloride coating over the substrate; (3) make the stannouschloride-containing coating more uniform in thickness; and (4)distribute the dopant component more uniformly in the stannouschloride-containing coating. Such maintaining preferably occurs for aperiod of time in the range of about 1 minute to about 20 minutes. Suchmaintaining is preferably conducted at the same or a higher temperaturerelative to the temperature at which the substrate/stannouschloride-containing composition contacting occurs. Such maintaining, ingeneral, acts to make the coating more uniform and, thereby provides forbeneficial electrical conductivity properties. The thickness of the tinoxide-containing coating is preferably in the range of about 0.1 micronto about 10 microns, more preferably about 0.25 micron to about 1.25microns.

The stannous chloride which is contacted with the substrate is in avaporous phase or state, or in a liquid phase or state at the time ofthe contacting. The composition which includes the stannous chloridepreferably also includes the fluorine component or components. Thiscomposition may also include one or more other materials, e.g., dopants,catalysts, grain growth inhibitors, solvents, etc., which do notsubstantially adversely promote the premature hydrolysis of the stannouschloride and/or the fluorine component, and do not substantiallyadversely affect the properties of the final product, such as by leavinga detrimental residue in the final product prior to the formation of thetin oxide-containing coating. Thus, it has been found to be important,e.g., to obtaining a tin oxide coating with good structural and/orelectronic properties, that undue hydrolysis of the stannous chlorideand fluorine component be avoided. This is contrary to certain of theprior art which actively utilized the simultaneous hydrolysis reactionas an approach to form the final coating. Examples of useful othermaterials include organic components such as acetonitrile, ethylacetate, dimethyl sulfoxide, propylene carbonate and mixtures thereof;certain inorganic salts and mixtures thereof. These other materials,which are preferably substantially anhydrous, may often be considered asa carrier, e.g., solvent, for the stannous chloride and/or fluorinecomponent to be contacted with the substrate. It has also been foundthat the substrate can first be contacted with a stannous chloridepowder, preferably with a film forming amount of stannous chloridepowder, followed by increasing the temperature to the liquidus point ofthe stannous chloride powder on the substrate to allow coating of, andpreferably equilibration on, the substrate. The size distribution of thestannous chloride powder and the amount of such powder applied to thesubstrate are preferably chosen so as to distribute the coating oversubstantially the entire substrate.

The stannous chloride and/or fluorine component to be contacted with thesubstrate may be present in a molten state. For example, a meltcontaining molten stannous chloride and/or stannous fluoride may beused. The molten composition may include one or more other materials,having properties as noted above, to produce a mixture, e.g., a eutecticmixture, having a reduced melting point and/or boiling point. The use ofmolten stannous chloride and/or fluorine component provides advantageoussubstrate coating while reducing the handling and disposal problemscaused by a solvent. In addition, the substrate is very effectively andefficiently coated so that coating material losses are reduced.

The stannous chloride and/or fluorine component to be contacted with thesubstrate may be present in a vaporous state. As used in this context,the term "vaporous state" refers to both a substantially gaseous stateand a state in which the stannous chloride and/or fluorine component arepresent as drops or droplets in a carrier gas, i.e., an atomized state.Liquid state stannous chloride and/or fluorine component may be utilizedto generate such vaporous state compositions.

In addition to the other materials, as noted above, the compositioncontaining stannous chloride and/or the fluorine component may alsoinclude one or more grain growth inhibitor components. Such inhibitorcomponent or components are present in an amount effective to inhibitgrain growth in the tin oxide-containing coating. Reducing grain growthleads to beneficial coating properties, e.g., higher electricalconductivity, more uniform morphology, and/or greater overall stability.Among useful grain growth inhibitor components are components whichinclude at least one metal, in particular potassium, calcium, magnesium,silicon and mixtures thereof. Of course, such grain growth inhibitorcomponents should have no substantial detrimental effect on the finalproduct.

The fluorine component may be deposited on the substrate separately fromthe stannous chloride, e.g., before and/or during and/or after thestannous chloride/substrate contacting. If the fluorine component isdeposited on the substrate separately from the stannous chloride, it ispreferred that the fluorine component be deposited after the stannouschloride.

Any suitable fluorine component may be employed in the present process.Such fluorine component should provide sufficient fluorine, e.g.,fluoride, dopant so that the final fluorine doped tin oxide coating hasthe desired properties, e.g., electronic conductivity, stability, etc.Care should be exercised in choosing the fluorine component orcomponents for use. For example, the fluorine component should besufficiently compatible with the stannous chloride so that the desiredfluorine doped tin oxide coating can be formed. Fluorine componentswhich have excessively high boiling points and/or are excessivelyvolatile (relative to stannous chloride), at the conditions employed inthe present process, are to be avoided since, for example, the finalcoating may not be sufficiently doped and/or a relatively large amountof the fluorine component or components may be lost during processing.It may be useful to include one or more property altering components,e.g., boiling point depressants, in the composition containing thefluorine component to be contacted with the substrate. Such propertyaltering component or components are included in an amount effective toalter one or more properties, e.g., boiling point, of the fluorinecomponent, e.g., to improve the compatibility or reduce theincompatibility between the fluorine component and stannous chloride.

Particularly useful fluorine components for use in the present inventionare selected from stannous fluoride, stannic fluoride, hydrogen fluorideand mixtures thereof. When hydrogen fluoride is used in the presentinvention, it has been found that excellent dopant incorporation isachieved. It is believed that the hydrogen fluoride is able to reactwith the tin chloride compound, for example, stannous chloride, to formtin fluoride, for example, stannous fluoride, which provides forsubstantial dopant incorporation. When stannous fluoride is used as afluorine component, it is preferred to use one or more boiling pointdepressants to reduce the apparent boiling point of the stannousfluoride, in particular to at least about 850° C. or less.

The use of a fluorine dopant is an important feature of certain aspectsof the present invention. First, it has been found that fluorine dopantscan be effectively and efficiently incorporated into the tinoxide-containing coating. In addition, such fluorine dopants act toprovide tin oxide-containing coatings with good electronic propertiesreferred to above, morphology and stability. This is in contrast tocertain of the prior art which found antimony dopants to be ineffectiveto improve the electronic . properties of tin oxide coatings.

The liquid, e.g., molten, composition which includes stannous chloridemay, and preferably does, also include the fluorine component. In thisembodiment, the fluorine component or components are preferably solublein the composition. Vaporous mixtures of stannous chloride and fluorinecomponents may also be used. Such compositions are particularlyeffective since the amount of dopant in the final doped tin oxidecoating can be controlled by controlling the make-up of the composition.In addition, both the stannous chloride and fluorine component aredeposited on the substrate in one step. Moreover, if stannous fluorideand/or stannic fluoride are used, such fluorine components provide thedopant and are converted to tin oxide during the oxidizingagent/substrate contacting step. This enhances the overall utilizationof the coating components in the present process. Particularly usefulcompositions comprise about 50% to about 98%, more preferably about 70%to about 95%, by weight of stannous chloride and about 2% to about 50%,more preferably about 5% to about 30%, by weight of fluorine component,in particular stannous fluoride.

In one embodiment, a vaporous stannous chloride composition is utilizedto contact the substrate, and the composition is at a higher temperaturethan is the substrate. The make-up of the vaporous stannouschloride-containing composition is such that stannous chloridecondensation occurs on the cooler substrate. If the fluorine componentis present in the composition, it is preferred that such fluorinecomponent also condense on the substrate. The amount of condensation canbe controlled by controlling the chemical make-up of the vaporouscomposition and the temperature differential between the composition andthe substrate. This "condensation" approach very effectively coats thesubstrate to the desired coating thickness without requiring that thesubstrate be subjected to numerous individual or separate contactingswith the vaporous stannous chloride-containing composition. As notedabove, previous vapor phase coating methods have often been handicappedin requiring that the substrate be repeatedly recontacted in order toget the desired coating thickness. The present "condensation" embodimentreduces or eliminates this problem.

The substrate including the stannous chloride-containing coating and thefluorine component-containing coating is contacted with an oxidizingagent at conditions effective to convert stannous chloride to tin oxide,preferably substantially tin dioxide, and form a fluorine doped tinoxide coating on at least a portion of the substrate. Water, e.g., inthe form of a controlled amount of humidity, is preferably presentduring the coated substrate/oxidizing agent contacting. The vapor can beadded at elevated temperatures as saturated or super saturated steam inorder to enhance overall conversion to tin oxide at reduced contactingtimes. This is in contrast with certain prior tin oxide coating methodswhich are conducted under anhydrous conditions. The presence of waterduring this contacting has been found to provide a doped tin oxidecoating having excellent electrical conductivity properties.

Any suitable oxidizing agent may be employed, provided that such agentfunctions as described herein. Preferably, the oxidizing agent (ormixtures of such agents) is substantially gaseous at the coatedsubstrate/oxidizing agent contacting conditions. The oxidizing agentpreferably includes reducible oxygen, i.e., oxygen which is reduced inoxidation state as a result of the coated substrate/oxidizing agentcontacting. More preferably, the oxidizing agent comprises molecularoxygen, either alone or as a component of a gaseous mixture, e.g., air.

The substrate may be composed of any suitable material and may be in anysuitable form. Preferably, the substrate is such so as to minimize orsubstantially eliminate the migration of ions and other species, if any,from the substrate to the tin oxide-containing coating which aredeleterious to the functioning or performance of the coated substrate ina particular application. In addition, it can be precoated to minimizemigration, for example a silica precoat and/or to improve wetability anduniform distribution of the coating materials on the substrate. In orderto provide for controlled electronic conductivity in the fluorine dopedtin oxide coating, it is preferred that the substrate be substantiallynon-electronically conductive when the coated substrate is to be used asa component of an electric energy storage battery. In one embodiment,the substrate is inorganic, for example glass and/or ceramic. Althoughthe present process may be employed to coat two dimensional substrates,such as substantially flat surfaces, it has particular applicability incoating three dimensional substrates. Thus, the present process is athree dimensional process. Examples of three dimensional substrateswhich can be coated using the present process include spheres,extrudates, flakes, single fibers, fiber rovings, chopped fibers, fibermats, porous substrates, irregularly shaped particles, e.g., catalystsupports, multi-channel monoliths and the like. Acid resistant glassfibers, especially woven and non-woven mats of acid resistant glassfibers, are particularly useful substrates when the fluorine doped tinoxide coated substrate is to be used as a component of a battery, suchas a lead-acid electrical energy storage battery. More particularly, thesubstrate for use in a battery is in the form of a body of woven ornon-woven fibers, still more particularly, a body of fibers having aporosity in the range of about 60% to about 95%. Porosity is defined asthe percent or fraction of void space within a body of fibers. Theabove-noted porosities are calculated based on the fibers including thedesired fluorine doped tin oxide coating.

The substrate for use in lead-acid batteries, because of availability,cost and performance considerations, preferably comprises acid resistantglass, more preferably in the form of fibers, as noted above.

The substrate for use in lead-acid batteries is acid resistant. That is,the substrate exhibits some resistance to corrosion, erosion and/orother forms of deterioration at the conditions present, e.g., at or nearthe positive plate, or positive side of the bipolar plates, in alead-acid battery. Although the fluorine doped tin oxide coating doesprovide a degree of protection for the substrate against theseconditions, the substrate should itself have an inherent degree of acidresistance. If the substrate is acid resistant, the physical integrityand electrical effectiveness of the doped tin oxide coating and of thewhole present battery element, is better maintained with time relativeto a substrate having reduced acid resistance. If glass is used as thesubstrate, it is preferred that the glass have an increased acidresistance relative to E-glass. Preferably, the acid resistant glasssubstrate is at least as resistant as is C-or T- glass to the conditionspresent in a lead-acid battery.

Typical compositions of E-glass and C-glass are as follows:

    ______________________________________                                                      Weight Percent                                                                E-glass C-glass T-glass                                         ______________________________________                                        Silica          54        65                                                  Alumina         14        4       6                                           Calcia          18        14      10*                                         Magnesia        5         3       --                                          Soda + Potassium Oxide                                                                        0.5       9       13                                          Boria           8         5       6                                           Titania + Iron Oxide                                                                          0.5       --      --                                          ______________________________________                                         *including MgO                                                           

Preferably the glass contains more than about 60% by weight of silicaand less than about 35% by weight of alumina, and alkali and alkalineearth metal oxides.

The conditions at which each of the steps of the present process occurare effective to obtain the desired result from each such step and toprovide a fluorine doped tin oxide coated substrate. Thesubstrate/stannous chloride contacting and the substrate/fluorinecomponent contacting preferably occur at a temperature in the range ofabout 250° C. to about 375° C., more preferably about 275° C. to about350° C. The amount of time during which stannous chloride and/orfluorine component is being deposited on the substrate depends on anumber of factors, for example, the desired thickness of the doped tinoxide coating, the amounts of stannous chloride and fluorine componentavailable for substrate contacting, the method by which the stannouschloride and fluorine component are contacted with the substrate and thelike. Such amount of time is preferably in the range of about 0.5minutes to about 20 minutes, more preferably about 1 minute to about 10minutes.

If the coated substrate is maintained in a substantially non-deleteriousoxidizing environment, it is preferred that such maintaining occur at atemperature in the range of about 275° C. to about 375° C., morepreferably about 300° C. to about 350° C. for a period of time in therange of about 0.1 minutes to about 20 minutes, more preferably about 1minute to about 10 minutes. The coated substrate/oxidizing agentcontacting preferably occurs at a temperature in the range of about 350°C. to about 600° C., more preferably about 400° C. to about 550° C., fora period of time in the range of about 0.1 minutes to about 10 minutes.A particular advantage of the process of this invention is thetemperatures used for oxidation have been found to be lower, in certaincases, significantly lower, i.e., 50° to 100° C. than the temperaturesrequired for spray hydrolysis. This is very significant and unexpected,provides for process efficiencies and reduces, and in some casessubstantially eliminates, migration of deleterious elements from thesubstrate to the tin oxide layer. Excessive sodium migration, e.g., fromthe substrate, can reduce electronic conductivity.

The pressure existing or maintained during each of these steps may beindependently selected from elevated pressures (relative to atmosphericpressure), atmospheric pressure, and reduced pressures (relative toatmospheric pressure). Slightly reduced pressures, e.g., less thanatmospheric pressure and greater than about 8 psia and especiallygreater than about 11 psia, are preferred.

The fluorine doped tin oxide coated substrate of the present inventionmay be, for example, a catalyst itself or a component of a compositetogether with one or more matrix materials. The composites may be suchthat the matrix material or materials substantially totally encapsulateor surround the coated substrate, or a portion of the coated substratemay extend away from the matrix material or materials.

Any suitable matrix material or materials may be used in a compositewith the fluorine doped tin oxide coated substrate. Preferably, thematrix material comprises a polymeric material, e.g., one or moresynthetic polymers, more preferably an organic polymeric material. Thepolymeric material may be either a thermoplastic material or a thermosetmaterial. Among the thermoplastics useful in the present invention arethe polyolefins, such as polyethylene, polypropylene, polymethylpenteneand mixtures thereof; and poly vinyl polymers, such as polystyrene,polyvinylidene difluoride, combinations of polyphenylene oxide andpolystyrene, and mixtures thereof. Among the thermoset polymers usefulin the present invention are epoxies, phenol-formaldehyde polymers,polyesters, polyvinyl esters, polyurethanes, melamine-formaldehydepolymers, and urea-formaldehyde polymers. Also included among the usefulpolymeric materials are natural and synthetic rubber materials, such asstyrene butadiene rubbers; acrylonitrile rubbers, such as acrylonitrilebutadiene styrene rubbers; ethylene propylene rubbers; chlorinatedderivatives thereof; and mixtures thereof.

When used in battery applications, the present doped tin oxide coatedsubstrate is preferably at least partially embedded in a matrixmaterial. The matrix material should be at least initially fluidimpervious to be useful in batteries. If the fluorine doped tin oxidecoated substrate is to be used as a component in a battery, e.g., alead-acid electrical energy storage battery, it is situated so that atleast a portion of it contacts the positive active electrode material.Any suitable positive active electrode material or combination ofmaterials useful in lead-acid batteries may be employed in the presentinvention. One particularly useful positive active electrode materialcomprises electrochemically active lead oxide, e.g., lead dioxide,material. A paste of this material is often used. If a paste is used inthe present invention, it is applied so that there is appropriatecontacting between the fluorine doped tin oxide coated substrate and thepaste.

In order to provide enhanced bonding between the fluorine doped tinoxide coated substrate and the matrix material, it has been found thatthe preferred matrix materials have an increased polarity, as indicatedby an increased dipole moment, relative to the polarity ofpolypropylene. Because of weight and strength considerations, if thematrix material is to be a thermoplastic polymer, it is preferred thatthe matrix be a polypropylene-based polymer which includes one or moregroups effective to increase the polarity of the polymer relative topolypropylene. Additive or additional monomers, such as maleicanhydride, vinyl acetate, acrylic acid, and the like and mixturesthereof, may be included prior to propylene polymerization to give theproduct propylene-based polymer increased polarity. Hydroxyl groups mayalso be included in a limited amount, using conventional techniques, toincrease the polarity of the final propylene-based polymer.

Thermoset polymers which have increased polarity relative topolypropylene are more preferred for use as the present matrix material.Particularly preferred thermoset polymers include epoxies,phenol-formaldehyde polymers, polyesters, and polyvinyl esters.

A more complete discussion of the presently useful matrix materials ispresented in Fitzgerald, et al U.S. Pat. No. 4,708,918, the entiredisclosure of which is hereby incorporated by reference herein.

Various techniques, such as casting, molding and the like, may be usedto at least partially encapsulate or embed the fluorine doped tin oxidecoated substrate into the matrix material or materials and formcomposites. The choice of technique may depend, for example, on the typeof matrix material used, the type and form of the substrate used and thespecific application involved. Certain of these techniques are presentedin U.S. Pat. No. 4,547,443, the entire disclosure of which is herebyincorporated by reference herein. One particular embodiment involvespre-impregnating (or combining) that portion of the doped tin oxidecoated substrate to be embedded in the matrix material with a relativelypolar (increased polarity relative to polypropylene) thermoplasticpolymer, such as polyvinylidene difluoride, prior to the coatedsubstrate being embedded in the matrix material. This embodiment isparticularly useful when the matrix material is itself a thermoplasticpolymer, such as modified polypropylene, and has been found to provideimproved bonding . between the fluorine doped tin oxide coated substrateand the matrix material.

The bonding between the matrix material and the fluorine doped tin oxidecoated, acid-resistant substrate is important to provide effectivebattery operation. In order to provide for improved bonding of thefluorine doped tin oxide coating (on the substrate) with the matrixmaterial, it is preferred to at least partially, more preferablysubstantially totally, coat the fluorine doped tin oxide coatedsubstrate with a coupling agent which acts to improve the bonding of thefluorine doped tin oxide coating with the matrix. This is particularlyuseful when the substrate comprises acid resistant glass fibers. Anysuitable coupling agent may be employed. Such agents preferably comprisemolecules which have both a polar portion and a non-polar portion.Certain materials generally in use as sizing for glass fibers may beused here as a "size" for the doped tin oxide coated glass fibers. Theamount of coupling agent used to coat the fluorine doped tin oxidecoated glass fibers should be effective to provide the improved bondingnoted above and, preferably, is substantially the same as is used tosize bare glass fibers. Preferably, the coupling agent is selected fromthe group consisting of silanes, silane derivatives, stannates, stannatederivatives, titanates, titanate derivatives and mixtures thereof. U.S.Pat. No. 4,154,638 discloses one silane-based coupling agent adapted foruse with tin oxide surfaces. The entire disclosure of this patent ishereby expressly incorporated by reference herein.

In the embodiment in which the fluorine doped tin oxide coated substrateis used as a component of a bipolar plate in a lead-acid battery, it ispreferred to include a fluid-impervious conductive layer that isresistant to reduction adjacent to, and preferably in electricalcommunication with, the second surface of the matrix material. Theconductive layer is preferably selected from metal, more preferablylead, and substantially non-conductive polymers, more preferablysynthetic polymers, containing conductive material. The non-conductivepolymers may be chosen from the polymers discussed previously as matrixmaterials. One particular embodiment involves using the same polymer inthe matrix material and in the conductive layer. The electricallyconductive material contained in the non-conductive layer preferably isselected from the group consisting of graphite, lead and mixturesthereof.

In the bipolar plate configuration, a negative active electrode layerlocated adjacent, and preferably in electric communication with, thefluid impervious conductive layer is included. Any suitable negativeactive electrode material useful in lead-acid batteries may be employed.One particularly useful negative active electrode material compriseslead, e.g., sponge lead. Lead paste is often used.

In yet another embodiment, a coated substrate including tin oxide,preferably electronically conductive tin oxide, and at least oneadditional catalyst component in an amount effective to promote achemical reaction is formed. Preferably, the additional catalystcomponent is a metal and/or a component of a metal effective to promotethe chemical reaction. The promoting effect of the catalyst componentmay be enhanced by the presence of an electrical field or electricalcurrent in proximity to the component. Thus, the tin oxide, preferablyon a substantially non-electronically conductive substrate, e.g., acatalyst support, can provide an effective and efficient catalyst forchemical reactions, including those which occur or are enhanced when anelectric field or current is applied in proximity to the catalystcomponent. Thus, it has been found that the present coated substratesare useful as active catalysts and supports for additional catalyticcomponents. Without wishing to limit the invention to any particulartheory of operation, it is believed that the outstanding stability,e.g., with respect to electronic properties and/or morphology and/orstability, of the present tin oxides plays an important role in makinguseful and effective catalyst materials. Any chemical reaction,including a chemical reaction the rate of which is enhanced by thepresence of an electrical field or electrical current as describedherein, may be promoted using the present catalyst component tinoxide-containing coated substrates. A particularly useful class ofchemical reactions are those involving chemical oxidation or reduction.For example, an especially useful and novel chemical reduction includesthe chemical reduction of nitrogen oxides, to minimize air pollution,with a reducing gas such as carbon monoxide, hydrogen and mixturesthereof and/or an electron transferring electrical field. Of course,other chemical reactions, e.g., hydrocarbon reforming, dehydrogenation,such as alkylaromatics to olefins and olefins to dienes,hydrodecyclization, isomerization, ammoxidation, such as with olefins,aldol condensations using aldehydes and carboxylic acids and the like,may be promoted using the present catalyst component, tinoxide-containing coated substrates. As noted above, it is preferred thatthe tin oxide in the catalyst component, tin oxide-containing substratesbe electronically conductive. Although fluorine doped tin oxide isparticularly useful, other dopants may be incorporated in the presentcatalyst materials to provide the tin oxide with the desired electronicproperties. For example, antimony may be employed as a tin oxide dopant.Such other dopants may be incorporated into the final catalystcomponent, tin oxide-containing coated substrates using one or moreprocessing techniques substantially analogous to procedures useful toincorporate fluorine dopant, e.g., as described herein.

Particularly useful chemical reactions as set forth above include theoxidative dehydrogenation of ethylbenzene to styrene and 1-butene to1,3-butadiene; the ammoxidation of propylene to acrylonitrile; aldolcondensation reactions for the production of unsaturated acids, i.e.,formaldehyde and propionic acid to form methacrylic acid andformaldehyde and acetic acid to form acrylic acid; the isomerization ofbutenes; and the oxidation of methane to methanol. It is believed,without limiting the invention to any specific theory of operation, thatthe stability of the catalysts, the redox activity of the tin oxide,i.e., stannous, stannic, mixed tin oxide redox couple, morphology andthe tin oxide catalytic and/or support interaction with other catalyticspecies provides for the making of useful and effective catalystmaterials. In certain catalytic reactions, such as NO_(x) reduction andoxidative dehydrogenation, it is believed that lattice oxygen from theregenerable tin oxide redox couple participates in the reactions.

The tin oxide-containing coated substrates of the present invention maybe employed alone or as a catalyst and/or support in a sensor, inparticular gas sensors. Preferably, the coated substrates includes asensing component similar to the catalyst component, as describedherein. The present sensors are useful to sense the presence orconcentration of a component, e.g., a gaseous component, of interest ina medium, for example, hydrogen, carbon monoxide, methane and otheralkanes, alcohols, aromatics, e.g., benzene, water, etc., e.g., byproviding a signal in response to the presence or concentration of acomponent of interest, e.g., a gas of interest, in a medium. Suchsensors are also useful where the signal provided is enhanced by thepresence of an electrical field or current in proximity to the sensingcomponent. The sensing component is preferably one or more metals ormetallic containing sensing components, for example, platinum,palladium, silver and zinc. The signal provided may be the result of thecomponent of interest itself impacting the sensing component and/or itmay be the result of the component of interest being chemically reacted,e.g., oxidized or reduced, in the presence of the sensing component.

The stability and durability for the present tin oxide materials arebelieved to make them very useful as catalysts, sensors, and supportsfor additional catalysts and sensors in aggressive and/or harshenvironments, particularly acid, i.e., sulfur and nitrogen acidenvironments.

Any suitable catalyst component (or sensing component) may be employed,provided that it functions as described herein. Among the useful metalcatalytic components and metal sensing components are those selectedfrom components of the transition metals, the rare earth metals, certainother catalytic components and mixtures thereof, in particular catalystscontaining gold, silver, copper, vanadium, chromium, tungsten, zinc,indium, antimony, the platinum group metals, i.e., platinum, palladium,iron, nickel, manganese, cesium, titanium, etc. Although metalcontaining compounds may be employed, it is preferred that the metalcatalyst component (and/or metal sensing component) included with thecoated substrate comprise elemental metal and/or metal in one or moreactive oxidized forms, for example, Cr₂ O₃, Ag₂ O, Sb₂ O₄, etc.

The preferred support materials include a wide variety of materials usedto support catalytic species, particularly porous refractory inorganicoxides. These supports include, for example, alumina, silica, zirconia,magnesia, boria, phosphate, titania, ceria, thoria and the like, as wellas multi-oxide type supports such as alumina-phosphorous oxide, silicaalumina, zeolite modified inorganic oxides, e.g., silica alumina, andthe like. As set forth above, support materials can be in many forms andshapes, especially porous shapes which are not flat surfaces, i.e., nonline-of-site materials. A particularly useful catalyst support is amulti channel monolith made from corderite which has been coated withalumina. The catalyst materials can be used as is or further processedsuch as by sintering of powered catalyst materials into largeraggregates. The aggregates can incorporate other powders, for example,other oxides, to form the aggregates.

The catalyst components (or sensing components) may be included with thecoated substrate using any one or more of various techniques, e.g.,conventional and well known techniques. For example, metal catalystcomponents (metal sensing components) can be included with the coatedsubstrate by impregnation; electrochemical deposition; spray hydrolysis;deposition from a molten slat mixture; thermal decomposition of a metalcompound or the like. The amount of catalyst component (or sensingcomponent) included is sufficient to perform the desired catalytic (orsensing function), respectively, and varies from application toapplication. In one embodiment, the catalyst component (or sensingcomponent) is incorporated while the tin oxide is placed on thesubstrate. Thus, a catalyst material, such as a salt or acid, e.g., ahalide and preferably chloride, oxy chloride and cloro acids, e.g.,chloro platinic acid, of the catalytic metal, is incorporated into thestannous chloride-containing coating of the substrate, prior to contactwith the oxidizing agent, as described herein. This catalyst materialcan be combined with the stannous chloride and contacted with thesubstrate, or it may be contacted with the substrate separately fromstannous chloride before, during and/or after the stannouschloride/substrate contacting.

The preferred approach is to incorporate catalyst-forming materials intoa process step used to form a tin oxide coating. This minimizes thenumber of process steps but also, in certain cases, produces moreeffective catalysts. The choice of approach is dependent on a number offactors, including the process compatibility of tin oxide andcatalyst-forming materials under given process conditions and theoverall process efficiency and catalyst effectiveness.

The tin oxide/substrate combinations, e.g., the tin oxide coatedsubstrates, of the present invention are useful in other applications aswell. Among these other applications are included porous membranes,resistance heating elements, electrostatic dissipation elements,electromagnetic interference shielding elements, protective coatings andthe like.

In one embodiment, a porous membrane is provided which comprises aporous substrate, preferably an inorganic substrate, and a tinoxide-containing material in contact with at least a portion of theporous substrate. In another embodiment, the porous membrane comprises aporous organic matrix material, e.g., a porous polymeric matrixmaterial, and a tin oxide-containing material in contact with at least aportion of the porous organic matrix material. With the organic matrixmaterial, the tin oxide-containing material may be present in the formof an inorganic substrate, porous or substantially non porous, having atin oxide-containing coating, e.g., an electronically conductive tinoxide-containing coating, thereon.

One particularly useful feature of the present porous membranes is theability to control the amount of tin oxide present to provide forenhanced performance in a specific application, e.g., a specificcontacting process. For example, the thickness of the tinoxide-containing coating can be controlled to provide such enhancedperformance. The coating process of the present invention isparticularly advantageous in providing such controlled coatingthickness. Also, the thickness of the tin oxide-containing coating canbe varied, e.g., over different areas of the same porous membrane, suchas an asymmetric porous membrane. In fact, the thickness of this coatingcan effect the size, e.g., diameter, of the pores. The size of the poresof the membrane or porous substrate may vary inversely with thethickness of the coating. The coating process of the present inventionis particularly useful in providing this porosity control.

A resistance heating element is provided which comprises a threedimensional substrate having an electronically conductive tinoxide-containing coating on at least a portion of all three dimensionsthereof. The coated substrate is adapted and structured to provide heatupon the application of an electrical potential across the coatedsubstrate. In one embodiment, a flexible heating element is providedwhich comprises a flexible matrix material, e.g., an organic polymericmaterial in contact with a substrate having an electronically conductivetin oxide-containing coating on at least a portion thereof. The coatedsubstrate is adapted and structured as described above.

In addition, an electrostatic dissipation/electromagnetic interferenceshielding element is provided which comprises a three dimensionalsubstrate, e.g., an inorganic substrate, having an electronicallyconductive tin oxide-containing coating on at least a portion of allthree dimensions thereof. The coated substrate is adapted and structuredto provide at least one of the following: electrostatic dissipation andelectromagnetic interference shielding. A flexible electrostaticdissipation/electromagnetic interference shielding element is alsoincluded in the scope of the present invention. This flexible elementcomprises a flexible matrix material, e.g., an organic polymericmaterial, in contact with a substrate having an electronicallyconductive tin oxide-containing coating on at least a portion thereof.The coated substrate of this flexible element is adapted and structuredas described above.

The present coating process is particularly suitable for controlling thecomposition and structure of the coating on the substrate to enhance theperformance of the coated substrate in a given, specific application,e.g., a specific resistance heating electrostatic dissipation orelectromagnetic interference shielding application.

The present tin oxide/substrate combinations and matrix material/tinoxide/substrate combinations, which have at least some degree ofporosity, hereinafter referred to as "porous contacting membranes" or"porous membranes", may be employed as active components and/or assupports for active components in systems in which the tinoxide/substrate, e.g., the tin oxide coated substrate, is contacted withone or more other components such as in, for example, separationsystems, gas purification systems, filter medium systems, flocculentsystems and other systems in which the stability and durability of suchcombinations can be advantageously utilized.

Particular applications which combine many of the outstanding propertiesof the products of the present invention include porous and electromembrane separations for food processing, textile/leather processing,chemical processing, bio medical processing and water treatment. Forexample, various types of solutions can be further concentrated, e.g.,latex concentrated, proteins isolated, colloids removed, salts removed,etc. The membranes can be used in flat plate, tubular and/or spiralwound system design. In addition, the products of this invention can beused e.g., as polymeric composites, for electromagnetic andelectrostatic interference shielding applications used for computers,telecommunications and electronic assemblies, as well as in low radarobservable systems and static dissipation, for example, in carpeting andin lightning protection systems for aircraft.

Membranes containing voids that are large in comparison with moleculardimensions are considered porous. In these porous membranes, the poresare interconnected, and the membrane may comprise only a few percent ofthe total volume. Transport, whether driven by pressure, concentration,or electrical potential . or field, occurs within these pores. Many ofthe transport characteristics of porous membranes are determined by thepore structure, with selectivity being governed primarily by therelative size of the molecules or particles involved in a particularapplication compared to the membrane pores. Mechanical properties andchemical resistance are greatly affected by the nature, composition andstructure e.g., chemical composition and physical state, of themembrane.

Commercial micropore membranes have pore dimensions, e.g., diameters, inthe range of about 0.005 micron to about 20 microns. They are made froma wide variety of materials in order to provide a range of chemical andsolvent resistances. Some are fiber or fabric reinforced to obtain therequired mechanical rigidity and strength. The operationalcharacteristics of the membrane are defined sometimes in terms of themolecules or particles that will pass through the membrane porestructure.

Microporous membranes are often used as filters. Those with relativelylarge pores are used in separating coarse disperse, suspendedsubstances, such as particulate contamination. Membranes with smallerpores are used for sterile filtration of gases, separation of aerosols,and sterile filtration of pharmaceutical, biological, and heat sensitivesolutions. The very finest membranes may be used to separate, e.g.,purify, soluble macromolecular compounds.

Porous membranes also are used in dialysis applications such a removingwaste from human blood (hemodialysis), for separation of biopolymers,e.g., with molecular weights in the range of about 10,000 to about100,000, and for the analytical measurements of polymer molecularweights. Microporous membranes also may be used as supports for verythin, dense skins or a containers for liquid membranes.

The ability of dense membranes to transport species . selectively makespossible molecular separation processes such as desalination of water orgas purification, but with normal thicknesses these rates are extremelyslow. In principle, the membranes could be made thin enough that therates would be attractive, but such thin membranes would be verydifficult to form and to handle, and they would have difficultysupporting the stresses imposed by the application. Conversely,microporous membranes have high transport rates but very poorselectivity for small molecules. Asymmetric membranes, for example madeof the present combinations, in which a very thin, dense membrane isplaced in series with a porous substructure are durable and provide highrates with high selectivity. Such asymmetric membranes and the usethereof are within the scope of the present invention.

Examples of applications for porous membranes include: separation offungal biomass in tertiary oil recovery; concentration of magnesiumhydroxide from sea water; concentration of PVC latex dispersions;separation of water/gasoline mixtures; particle removal from hot coalgasification products; desalination of sea water; enhancement ofcatecholamine determination; removal of colloids from high puritydeionized water; treatment of wool scouring liquids; filtration oftissue homogenates; separation of antigen from antigen-antibody couplein immunoassay; purification of subcutaneous tissue liquid extracts;concentration of solubilized proteins and other cellular products; celldebris removal; concentration of microbial suspensions (microbialharvesting); enzyme recovery; hemodialysis; removal of casein, fats andlactose from whey; concentration of albumen; separation of skimmed milk;clarification of liqueur, fruit juices, sugar, and corn syrup; alcoholfermentation; sterilization of liquids, e.g., beer, wine; continuousmicrofiltration of vinegar; concentration and demineralization ofcheese, whey, soy whey, vegetable extracts, and flavorings; sugar wasterecovery; silver recovery from photorinses; dewatering of hazardouswastes; removal of suspended solids from lead-acid battery manufacture;removal of cyanides from electroplating waste waters; removal andrecovery of catalyst fines; removal of radionuclides and metal hydroxideprecipitates; recovery of commercial asbestos from asbestos-cementindustry waste waters; re-use of waste water containing fire-fightingagent; removal of hydrocarbon oils from waste water; recovery andrecycling of sewage effluent; recovery of dye stuffs from textile millwastes; recovery of starch and proteins from factory waste, wood pulp,and paper processing; separation of water and oil emulsions; separationof carbon dioxide and methane; and catalytic chemical reactions.

As described above porous membranes can be used in a wide variety ofcontacting systems. In a number of applications, the porous membraneprovides one or more process functions including: filtration,separation, purification, recovery of one or more components, emulsionbreaking, demisting, floculation, resistance heating and chemicalreaction (catalytic or noncatalytic), e.g., pollutant destruction to anon-hazardous form. The resistance heating and chemical reactionfunctions (applications) set forth herein can be combined with one ormore other functions set forth herein for the porous membranes as wellas such other related porous membrane applications.

The porous membrane, in particular the substrate, can be predominatelyorganic or inorganic, with an inorganic substrate being suitable fordemanding process environments. The porous organic-containing membranesoften include a porous organic based polymer matrix material havingincorporated therein a three dimensional tin oxide-containing material,preferably including an electronically conductive tin dioxide coating,more preferably incorporating a dopant and/or a catalytic species, in anamount that provides the desired function, particularly electricalconductivity, without substantially deleteriously affecting theproperties of the organic polymer matrix material. These modifiedpolymer membranes are particularly useful in porous membrane and/orelectromembrane and/or catalytic processes.

Examples of polymer materials useful in microporous membranes includecellulose esters, poly(vinyl chloride), high temperature aromaticpolymers, polytetrafluoroethylene, polymers sold by E.I. DuPontCorporation under the trademark Nafion, polyethylene, polypropylene,polystyrene, polyethylene, polycarbonate, nylon, silicone rubber, andasymmetric coated polysulfone fiber.

A very convenient application for the coating process and products ofthis invention is the production of a controlled coating, e.g, a thincoating of tin oxide-containing material, on an inorganic substrate,particularly a porous inorganic substrate, to produce a porous membrane.The process provides a new generation of membranes: porous membranes forcontacting processes, e.g., as described herein. The selectivity infiltration, particularly ultra and micro filtration, can also beenhanced by applying an electrical field and/or an electrical potentialto the porous membrane. The electrical field and/or potential can beobtained using a two electrode electrical system, the membrane includinga electronically conductive tin oxide-containing coating constitutingone of the two electrodes (anode or cathode).

Porous multilayer asymmetric electronically conductive inorganicmembranes, produced in accordance with this invention, are particularlyadvantageous for membrane applications. Among the advantages of suchmembranes are: stability at high temperature and/or at large pressuregradients, mechanical stability (reduced and even substantially nocompaction of the membrane under pressure), stability againstmicrobiological attack, chemical stability especially with organicsolvents, steam sterilization at high temperatures, backflush cleaningat pressures of up to 25 atm, and stability in corrosive and oxidationenvironment.

A membrane can be classified as a function of the size of the particles,macromolecules and molecules separated. Micron sized porous ceramics forfiltration processes can be prepared through sintering of appropriatematerials as set forth herein for the manufacture of sensors. However,the preferred process for membrane-based microfiltration,ultrafiltration and reverse osmosis is to provide inorganic layers withultrafine pores and thickness small enough to obtain high flux throughthe membrane, particularly membranes including tin oxide-containingcoatings.

With this type of asymmetric membrane, separation processes are pressuredriven. Another factor is the interaction at the membrane interfacebetween the porous material and the material to be processed. As notedabove, selectivity can be enhanced by applying an electrical field ontothe surface of the membrane. The electrical field is obtained using atwo electrode electrical device; the conductive membrane constitutingone of the two electrodes (anode or cathode - preferably anode). Suchporous membranes can be obtained with one or more electronicallyconductive tin oxide-containing thin layers on a porous substrate.Conductive tin oxide combined with other metal oxide mixtures alsoprovide improved properties for porous membranes and exhibit electronicconductivity, as well as other functions, such as catalysis orresistance heating.

As set forth above, porous membranes with inorganic materials can beobtained through powder agglomeration, the pores being the intergranularspaces. Conflicting requirements such as high flow rate and mechanicalstability can be achieved using an asymmetric structure. Thus, aninorganic porous membrane is obtained by superimposing a thinmicroporous film, which has a separative function, over a thickmacroporous support. For . example, conductive tin oxide coating ontothe surface of filter media can be used as well as onto the surface offlat circular alumina plates. Coated alumina membranes supported on theinner part of sintered alumina tubes designed for industrialultrafiltration processes can be used. Tube-shaped supports can be usedwith varying different chemical compositions, such as oxides, carbides,and clays. Coating of a homogeneous and microporous tin oxide-containinglayer depends on surface homogeneity of the support and on adherencebetween the membrane and its support. Superior results can be obtainedwith particulate alumina. The inner part of the tube has a membranecomprising a layer, e.g., in the range of about 10 to about 20 micronsthick, with pores, e.g., having diameters in the range of about 0.02 toabout 0.2 microns sized for microfiltration purposes. The main featureof such a membrane is uniform surface homogeneity allowing for the tinoxide-containing coating to be very thin, e.g., less than about onemicron in thickness.

The products of this invention, as described herein, are particularlyuseful for resistance heating applications. It has been found that thecoated three dimensional and/or flexible substrates particularly fibers,fiber rovings, chopped fibers, and fiber mats, can be incorporated intopolymeric matrix materials, particularly thermoplastic, thermoset andrubber based polymeric materials, as described herein. The tin oxidecoated substrates can be, for example, E, C, S, or T glass, silica,silica alumina, silica alumina boria, silicon carbide or alumina fibers,rovings, mats, chopped mats, etc. What is unexpected is the improvedmechanical properties, e.g., strength, coating adhesion and the like, ofthe coated substrates relative to the prior art substrates coated usingspray pyrolysis techniques and the improved control over coatingthickness to match conductivity requirements for a given resistanceheating application. Whereas for many low to moderate temperatureapplications, organic polymer matrix materials are preferred, threedimensional products comprising, preferably primarily comprisingflexible or rigid inorganic substrates coated with tin oxide-containingcoatings have excellent high temperature performance characteristicsuseful, for example, in high temperature resistance heating of liquidsand gases, such as air, by contact with or through (i.e., porous) suchthree dimensional products. Typical resistance heating applicationsinclude: heating elements or units of electric heating devices, devicesfor culinary purposes, warming tables, therapeutic heaters, deicingdevices such as electrically heated polymer composites, low-temperatureovens such as driers, high temperature heating of gases, liquids, etc.

Another unique application of the present invention combines thestability of the tin oxide-containing coating, particularly at hightemperatures and/or in demanding oxidizing environments, with the needto protect a structural element and/or to provide a fluid, i.e., gasand/or liquid, impervious material. Such structural elements aresuitable for use at high temperatures, preferably greater than about400° F., more preferably greater than about 1500° F. or even greaterthan about 2000° F. The present coatings preferably provide protectionagainst oxidation. Examples of structural elements requiring suchprotection and/or a fluid impervious coating include three dimensionalsubstantially carbon or inorganic materials, such as woven ceramicfibers and carbon-carbon composites, useful as turbine enginecomponents, hot air frame components, and hypersonic vehicle structuralelements or components. Due to the fact that carbon oxidizes under thedemands of such environments, barrier or protective coatings arenecessary. A particularly effective barrier coating is a tinoxide-containing coating formed according to the present inventionbecause of the high temperature stability and excellent and completecoverage of such coating.

In addition, it is believed that a layer of at least one lower valenceoxide of tin may form at the carbon tin oxide interface thereby givingadditional barrier protection against excessive carbon oxidation tocarbon oxides gases and decomposition products. The coating process ofthis invention, in addition, can uniformly coat three dimensional wovenstructures, particularly in the vaporous state, to effectively seal offdiffusion of gases and/or liquids between surfaces. For example, ceramicfibers, such as those sold under the trademark Nextel by the 3M Company,can be woven into structures or structural elements, sealed off betweensurfaces, and used in high temperature applications. Such applicationsinclude gas and/or oil radiant and post combustion burner tubes, turbineengine components, and combustion chambers. For the latter, suchstructures can also contain one or more catalytically active materialsthat promote combustion, such as hydrocarbon combustion.

A particularly unique application that relies upon stable electronicconductivity and the physical durability of the products of thisinvention are dispersions of conductive material, such as powders, influids, e.g., water, hydrocarbons, e.g., mineral or synthetic oils,whereby an increase in viscosity, to even solidification, is obtainedwhen an electrical field is applied to the system. These fluids arereferred to as "field dependent" fluids which congeal and which canwithstand forces of shear, tension and compression. These fluids revertto a liquid state when the electric field is turned off. Applicationsinclude dampening, e.g., shock absorbers, variable speed transmissions,clutch mechanisms, etc.

Certain of these and other aspects the present invention are set forthin the following description of the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block flow diagram illustrating one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWING

The following description specifically involves the coating of randomlyoriented, woven mats of C-glass fibers. However, it should be noted thatsubstantially the same process steps can be used to coat other substrateforms and/or materials.

A process system according to the present invention, shown generally at10, includes a preheat section 12, a coating section 14, anequilibration section 16 and an oxidation/sintering section 18. Each ofthese sections is in fluid communication with the others. Preferably,each of these sections is a separate processing zone or section.

First gas curtain 20 and second gas curtain 22 provide inert gas,preferably nitrogen, at the points indicated, and, thereby effectivelyinsure that preheat section 12, coating section 14 and equilibriumsection 16 are maintained in a substantially inert environment. Firstexhaust 24 and second exhaust 26 are provided to allow vapors to exit orbe vented from process system 10.

Randomly oriented woven mats of C-glass fibers from substrate source 28are fed to preheat section 12 where the mats are preheated up to amaximum of 375° C. for a time of 1 to 3 minutes at atmospheric pressureto reach thermal equilibrium. These mats are composed of from 8 micronto 35 micron diameter C-or T-glass randomly oriented or woven fibers.The mats are up to 42 inches wide and between 0.058 to 0.174 mil thick.The mats are fed to process system 10 at the rate of about 1 to 5 feetper minute so that the fiber weight throughout is about 0.141 to about2.1 pounds per minute.

The preheated mats pass to the coating section 14 where the mats arecontacted with an anhydrous mixture of 70% to 95% by weight of stannouschloride and 5% to 30% by weight of stannous fluoride from raw materialsource 30. This contacting effects a coating of this mixture on themats.

This contacting may occur in a number of different ways. For example,the SnCl₂ /SnF₂ mixture can be combined with nitrogen to form a vaporwhich is at a temperature of from about 25° C. to about 150° C. higherthan the temperature of the mats in the coating section 14. As thisvapor is brought into contact with the mats, the temperaturedifferential between the mats and the vapor and the amount of themixture in the vapor are such as to cause controlled amounts of SnCl₂and SnF₂ to condense on and coat the mats.

Another approach is to apply the SnCl₂ /SnF₂ mixture in a molten formdirectly to the mats in an inert atmosphere. There are severalalternatives for continuously applying the molten mixture to the mats.Obtaining substantially uniform distribution of the mixture on the matsis a key objective. For example, the mats can be compressed between tworollers that are continuously coated with the molten mixture. Anotheroption is to spray the molten mixture onto the mats. The fiber mats mayalso be dipped directly into the melt. The dipped fiber mats may besubjected to a compression roller step, a vertical lift step and/or avacuum filtration step to remove excess molten mixture from the fibermats.

An additional alternative is to apply the SnCl₂ /SnFn₂ in an organicsolvent. The solvent is then evaporated, leaving a substantially uniformcoating of SnCl₂ /SnF₂ on the fiber mats. The solvent needs to besubstantially non-reactive (at the conditions of the present process)and provide for substantial solubility of SnCl₂ and SnF₂. For example,the dipping solution involved should preferably be at least about 0.1molar in SnCl₂. Substantially anhydrous solvents comprisingacetonitrile, ethyl acetate, dimethyl sulfoxide, propylene carbonate andmixtures thereof are suitable. Stannous fluoride is often less solublein organic solvents than is stannous chloride. One approach toovercoming this relative insolubility of SnF₂ is to introduce SnF₂ ontothe fiber mats after the fiber mats are dipped into the SnCl₂ solutionwith organic solvent. Although the dopant may be introduced in thesintering section 18, it is preferred to incorporate the dopant in thecoating section 14 or the equilibration section 16, more preferably thecoating section 14.

Any part of process system 10 that is exposed to SnCl₂ and/or SnF₂ meltor vapor is preferably corrosion resistant, more preferably lined withinert refractory material.

In any event, the mats in the coating section 14 are at a temperature ofup to about 375° C., and this section is operated at slightly less thanatmospheric pressure. If the SnCl₂ /SnF₂ coating is applied as a moltenmelt between compression rollers, it is preferred that such compressionrollers remain in contact with the fiber mats for about 0.1 to about 2minutes, more preferably about 1 to about 2 minutes.

After the SnCl₂ /SnF₂ coating is applied to the fiber mats, the fibermats are passed to the equibration section 16. Here, the coated fibermats are maintained, preferably at a higher temperature than in coatingsection 14, in a substantially inert atmosphere for a period of time,preferably up to about 10 minutes, to allow the coating to moreuniformly distribute over the fibers. In addition, if the fluorinecomponent is introduced onto the fiber mats separate from the stannouschloride, the time the coated fiber mats spend in the equilibrationsection 16 results in the dopant component becoming more uniformlydispersed or distributed throughout the stannous chloride coating.Further, it is preferred that any vapor and/or liquid which separatefrom the coated fiber mats in the equilibration section 16 betransferred back and used in the coating section 14. This preferredoption, illustrated schematically in FIG. 1 by lines 32 (for the vapor)and 34 (for the liquid) increases the effective overall utilization ofSnCl₂ and SnF₂ in the process so that losses of these components, aswell as other materials such as solvents, are reduced.

The coated fiber mats are passed from the equilibration zone 16 into thesintering zone 18 where such fiber mats are contacted with an oxidizer,such as an oxygen-containing gas, from line 36. The oxidizer preferablycomprises a mixture of air and water vapor. This mixture, whichpreferably includes about 1% to about 50%, more preferably about 15% toabout 35%, by weight of water, is contacted with the coated fiber matsat atmospheric pressure at a temperature of about 400° C. to about 550°C. for up to about 10 minutes. Such contacting results in converting thecoating on the fiber mats to a fluorine doped tin dioxide coating. Thefluorine doped tin oxide coated fiber mats product, which exitssintering section 18 via line 38, has useful electric conductivityproperties. This product preferably has a doped tin oxide coating havinga thickness in the range of about 0.5 microns to about 1 micron, and isparticularly useful as a component in a lead-acid battery. Preferably,the product is substantially free of metals contamination which isdetrimental to electrical conductivity.

The present process provides substantial benefits. For example, theproduct obtained has a fluorine doped tin oxide coating which has usefulproperties, e.g., outstanding electrical and/or morphologicalproperties. This product may be employed in a lead-acid battery or incombination with a metallic catalyst to promote chemical reactions,e.g., chemical reductions, or alone or in combination with a metallicsensing component to provide sensors, e.g., gas sensors. Highutilization of stannous chloride and fluorine components is achieved. Inaddition, high coating deposition and product throughput rates areobtained. Moreover, relatively mild conditions are employed. Forexample, temperatures within sintering section 18 can be significantlyless than 600° C. The product obtained has excellent stability anddurability.

The following non-limiting example illustrates certain aspects of thepresent invention.

EXAMPLE

A substrate made of C-glass was contacted with a molten mixturecontaining 30 mol% SnF₂ and 70 mol% SnCl₂. This contacting occurred at350° C. in an argon atmosphere at about atmospheric pressure andresulted in a coating containing SnCl₂ and SnF₂ being placed on thesubstrate.

This coated substrate was then heated to 375° C. and allowed to stand inan argon atmosphere at about atmospheric pressure for about 5 minutes.The coated substrate was then fired at 500° C. for 20 minutes usingflowing, i.e., at the rate of one (1) liter per minute, water saturatedair at about atmospheric pressure. This resulted in a substrate having afluorine doped tin oxide coating with excellent electronic properties.For example, the volume resistivity of this material was determined tobe 7.5 ×10⁻⁴ ohm-cm.

In the previously noted publication "Preparation of Thick CrystallineFilms of Tin Oxide and Porous Glass Partially Filled with Tin Oxide andPorous Glass Partially Filled with Tin Oxide", an attempt to produceantimony doped tin oxide films on a 96% silica glass substrate involvingstannous chloride oxidation at anhydrous conditions resulted in amaterial having a volume resistivity of 1.5×10⁷ ohm-cm.

The present methods and products, illustrated above, provide outstandingadvantages. For example, the fluorine doped tin oxide coated substrateprepared in accordance with the present invention has improved, i.e.,reduced, electronic resistivity, relative to substrates produced byprior methods.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

What is claimed is:
 1. A process for separating a salt from a saltcontaining solution comprising contacting said salt containing solutionwith a porous contacting membrane comprising a material selected fromthe group consisting of a porous inorganic substrate in combination withan electronically conductive tin oxide containing material and a porousorganic material in combination with an inorganic substrate at leastpartially coated with an electronically conductive tin oxide containingmaterial at conditions effective to separate said salt containingsolution into a more concentrated salt solution and a more lean saltsolution.
 2. The process of claim 1 wherein said tin oxide containingmaterial is present in the form of a tin oxide containing coating on atleast a portion of said porous inorganic substrate.
 3. The process ofclaim 2 wherein said porous contacting membrane is structured as anasymmetric membrane.
 4. The process of claim 3 wherein the saltcontaining solution is seawater.
 5. The process of claim 2 wherein theporous contacting membrane is structured as an electro membrane.
 6. Theprocess of claim 2 wherein the porous contacting membrane is a porousorganic material.
 7. The process of claim 6 wherein the organic materialis a polymer material selected from the group consisting of celluloseesters, poly(vinyl chloride), high temperature aromatic polymers,polytetrafluoroethylene, sulfonic acid modified polymers ofpolytetrafluoroethylene, polyethylene, polypropylene, polystyrene,polyethylene, polycarbonate, nylon, silicone rubber and polysulfone. 8.The process of claim 7 wherein the salt containing solution is seawater.9. The process of claim 8 wherein the electronically conductive tinoxide is a fluoride dopes electronically conductive tin oxide.
 10. Theprocess of claim 6 wherein the inorganic substrate is porous.
 11. Theprocess of claim 6 wherein the porous contacting membrane is structuredas an asymmetric membrane.
 12. The process of claim 6 wherein theinorganic substrate is in a form selected from the group consisting ofspheres, extrudates, flakes, single fibers, chopped fibers, poroussubstrates, irregularly shaped particles and mixtures thereof.
 13. Theprocess of claim 12 wherein the electronically conductive tin oxide is afluoride doped electronically conductive tin oxide.
 14. The process ofclaim 6 wherein the salt containing solution is seawater.
 15. Theprocess of claim 1 wherein said porous inorganic substrate and saidinorganic substrate are three-dimensional.
 16. The process of claim 15wherein the thickness of said tin oxide coating on the porous contactingmembrane is controlled to provide for enhanced performance in theseparating process.
 17. The process of claim 15 wherein the porouscontacting membrane is structured as an asymmetric membrane.
 18. Theprocess of claim 17 wherein the porous contacting membrane is structuredas an asymmetric membrane.
 19. The process of claim 17 wherein theinorganic substrate is alumina.
 20. The process of claim 17 wherein thesalt containing solution is seawater.
 21. The process of claim 7 whereinthe electronically conductive tin oxide is a fluoride dopedelectronically conductive tin oxide.
 22. The process of claim 15 whereinthe electronically conductive tin oxide is a fluoride dopedelectronically conductive tin oxide.
 23. The process of claim 1 whereinthe salt containing solution is seawater.